(a) Field of the Invention
The present invention relates to a film mode detection method, and in particular, to a method for detecting whether a transferred video sequence is a 3:2 pull-down video sequence (film mode) or not, using a periodic pattern of the video sequence.
(b) Description of the Related Art
Generally, a movie film comprises 24 frames per second (FPS). Accordingly, in order to display the film by a TV system, the 24 FPS of motion picture signals must be converted to an appropriate frame rate matched for the TV system. For example, to display a film in an NTSC TV system, the 24 FPS of film signals are converted into 30 FPS of NTSC signals. A device called Telecine is used for this purpose of converting a sequence of a frame rate to a sequence of another frame rate.
In converting the 24 FPS of film signals into 30 FPS of NTSC signals, the Telecine uses a 3:2 pull-down process, which is called in a film mode because it is applied to the movie film. The 3:2 pull-down process converts the film signal to NTSC signals in such a way of converting two frames of 24 Hz into 5 fields of 60 Hz. In other words, three fields are made by repeating one frame out of the first 24 Hz frame, and two fields are made out of the second 24 Hz frame, accordingly 5 fields of the 60 Hz are made out of the 2 frames of 24 Hz.
Also, the digital video disc (DVD) emerged in place of the VCR tape adopts the 30 Hz frame frequency for signal processing of NTSC system and for maintaining a compatibility with it. Thus, the 3:2 pull-down process is usually used in a video system using digital storage and reproduction media such as the DVD.
In the case of ATSC system, since there exists a video format having 24 Hz frame frequency, the 24 FPS film can be coded in 24P format (24 Hz frame frequency, progressive scanning). However, in a position of a broadcasting station using 60I format (30 Hz frame frequency, 2:1 interlaced scanning) for normal television broadcasting, it can be a burden to accommodate the two kinds of formats of 24P and 60I.
In view of the above situation, the film signal adopting 3:2 pull-down process to support the 60I format using 30 Hz frame frequency will be remained as a dominant source of video sequence even in a time when the DVD and digital broadcasting are universalized.
Accordingly, it is very important to detect the 3:2 pull-down film mode from signals to be decoded.
It is an object of the present invention to provide an effective method for detecting a film mode which is capable of simply determining whether input signals are 3:2 pull-down signals.
To achieve the above object, the film mode detection method according to the present invention uses a periodic pattern of a video sequence. The method according to the present invention comprises the steps of: (a) computing every absolute value of differences between every pair of corresponding pixels in two adjacent fields which are arranged within a predetermined distance on a time line; (b) comparing a sum of the absolute value of differences between every corresponding pixels in two pixels with a first predetermined threshold value M1 and limiting the sum value to the first threshold value if the sum is greater than the first threshold value; (c) filtering the video sequence comprising the sum of the absolute value of differences between every pair of pixels in two fields and calculating a power of the filtered sum value; and (d) comparing the calculated power to a second threshold value M2 so as to determine the signal as a film mode if the power of the sum value is greater than the second threshold value.